Color image processing apparatus and color printer system

ABSTRACT

A color image processing apparatus includes: a minimum value selection unit that selects a minimum value of an input gray level values; a first calculation unit that calculates a generation amount of black; a second calculation unit that calculates an UCR amount which is a value relevant to the minimum value; a third calculation unit that outputs a correction coefficient value which is a value relevant to the minimum value; a subtraction unit that subtracts the UCR amount from each of the input gray level values to generate output; a multiplication unit that multiply the output of the subtraction unit by the correction coefficient value; a significant bit selection unit that selects a significant bit of a multiplication result; and a gray level correction unit that makes a gray level correction to the generation amount of black and the output of the multiplication unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a color image processing apparatus and a colorprinter system and in particular to a color image processing apparatusand a color printer system for generating image signals of four colorsof cyan (C), magenta (M), yellow (Y), and black (K) from red (R), green(G), and blue (B) image signals.

2. Description of the Related Art

Although the basic primary colors in color print are three colors ofcyan (C), magenta (M), and yellow (Y), a printer with four colors of C,M, Y and black (K) as the basic primary colors is often used from theviewpoint of the quality of frequently used black font, the print cost,or the advantage in a clear natural image.

On the other hand, input image data of a business printer, etc., used inan office generally is provided by three-color data of red (R), green(G), and blue (B) and therefore four-color separation of a process ofgenerating CMYK data from RGB data is required.

Usually the four-color separation is implemented as processing ofcalculating the complement of RGB data (r, g, b), generating CMY data(c, m, y), and partially replacing CMY common element min {c, m, y} withK component data (k) where min {c, m, y} is the minimum value of the CMYdata (c, m, y), as disclosed on page 476 of “PostScript LanguageReference third edition” (Adobe Systems Incorporated, February 1999).

The K component generated by performing the processing as describedabove is called BG (Black Generation) and the CMY component removed asthe CMY data is replaced with K is called UCR (Under Color Removal).

The effect of decreasing the maximum color material amount per unitprint area is shown as one of the large merits of such BG processing andUCR processing. In the Specification, the maximum color material amountper unit print area will be hereinafter called simply the maximum colormaterial amount or if the printer is a laser printer, the maximum toneramount.

For example, a method called 100% UCR is available as a method capableof most decreasing the maximum color material amount. This is a methodof subtracting k found as k=min {c, m, y} from CMY data (c, m, y) toobtain CMY data (c′, m′, y′) after undergoing the UCR processing as inexpression (1): $\begin{matrix}\left. \begin{matrix}{\quad{c^{\quad\prime}\quad = \quad{c\quad - \quad k}}} \\{\quad{m^{\quad\prime}\quad = \quad{m\quad - \quad k}}} \\{\quad{y^{\quad\prime}\quad = \quad{y\quad - \quad k}}}\end{matrix} \right\} & (1)\end{matrix}$

The maximum color material amount of four-color print of CMYK in the100% UCR becomes 200% of the maximum color material amount per color.Since the maximum color material amount to print in four colors of CMYKwithout performing UCR processing is 400%, it is seen that the maximumcolor material amount can be adjusted in the range of 200% to 400%byperforming BG processing and UCR processing.

On the other hand, generally as for printers for printing in multiplecolors, it is known that the more excessive the maximum color materialamount, the larger the apparatus load, resulting in a tendency towardthe instability of reproducibility of overlaid colors.

For example, in a laser printer, as the toner layer is thicker, thetransfer performance becomes more easily unstable and in an ink jetprinter, as the ink amount is larger, ink bleeding to paper becomeseasier to occur. In a laser printer for fusing toner, if an attempt ismade to cover the maximum toner amount, the heat energy to be input in aunit time becomes large and thus problems of upsizing of a fuser andshortening of the fuser life are introduced.

Thus, to develop a printer engine, requirement for cutting down themaximum color material amount as long as the image quality is allowedoccurs. However, from the viewpoint of the image quality, if extreme UCRprocessing like 100% UCR is performed, an extremely hard and dark imagelow in saturation on the whole is produced and the contrast is degradedbecause the color material amount is small.

From the contrary demands, the value to provide the best balance betweenthe engine performance and the image quality is selected for design inthe range of about 300% to 350% per color for commercial print or in therange of about 200% to 300% in a popularly priced laser printer, etc.,for limitation of the maximum color material amount.

On the other hand, an art of attaching a density sensor to a printer andmaking gray level correction using a detection signal from the sensor,thereby controlling the color material amount is also already known.

For example, JP-A-10-39555 discloses an art of optimizing a look-uptable (LUT) to convert the density data for each gray level into anoutput signal by performing first gray level control and controlling thetoner replenishment amount for stabilizing the image density byperforming second gray level control to keep the gray levelcharacteristic constant against environment fluctuation. However, thisexample concerns stabilization of the single-color gray levelcharacteristic and does not provide means for circumventing imageinstability caused by excess of the total toner amount in color print asdescribed above.

JP-A-5-328131 discloses an art of managing the toner deposition amountfor adjusting the density constant so that the density of the wholeoutput image does not change if the user adjusts the gray levelcharacteristic.

Further, JP-A-2001-255711 proposes an art of calculating the tonerdeposition amount of an image based on the gray level data subjected togray level correction and measurement data of a temperature-humiditysensor and alerting the user by notification means if the calculationresult is equal to or greater than the upper limit value of the tonerdeposition amount determined by the fixing performance of a fuser.

The arts in JP-A-5-328131 and JP-A-2001-255711 assume that hardwarefeedback is executed in response to user's adjustment. Thus, in aprinter shared among computers, a sequence for feedback occurs each timea print request is made, resulting in extra load on the apparatus and anincrease in the operation cost.

JP-A-2003-312061 discloses a method of directly limiting the sum totalof color components using a multi-dimensional LUT. In such a method,however, if the number of lattice points is five in a LUT correspondingto four-dimensional input/output of CMYK, as many as 5⁴×4=2500 pieces ofdata are required.

Use of such a large-scaled multi-dimensional LUT leads to an increase incost because a large memory space is consumed when the LUT is installedin hardware. If an LUT needs to be again set from the beginning, forexample, as the image processing mode is switched between print pages,etc., the time required for transferring the LUT data increases and thusan obstacle to speeding up is presented. A similar problem also occursin JP-A-2000-25274 and JP-A-2000-343761 because a multi-dimensional LUTis used.

To use a multi-dimensional LUT, calculation (or signal processing) loadof interpolation calculation also occurs. For example, if interpolationcalculation of comparatively simple multiple linear interpolation isperformed for finding, when the interpolation value is calculated fromthe vertices of eight peripheral grids as in FIG. 3 in JP-A-2000-343761,as many multiplications as (number of input channels)×(number ofvertices)×(number of output channels) become necessary.

In this case, the number of input channels is four for CMYK, the numberof vertices is eight (interpolation using eight peripheral vertices inblack point 11 in FIG. 3 in JP-A-2000-343761), and the number of outputchannels is four (CMYK) and therefore the necessary number ofmultiplications becomes 128. Thus, in hardware implementation, the logicscale grows and in software implementation, the computation amountbecomes an obstacle to speeding up.

JP-A-2003-125225 discloses a method of feeding back so that the totalamount becomes a preset value or less for each input of each colorsignal. In this method, iteration of processing caused by feeding backoccurs for each pixel and thus the method is disadvantageous to speedingup of real-time image processing. In contrast, the saturation andcontrast of an image would be able to be improved by performingprocessing of increasing the total amount in the range in which thetotal amount does not exceed the preset value, but the method inJP-A-2003-125225 cannot be applied to the processing of increasing thetotal amount.

JP-A-2005-33348 discloses a method of increasing the toner total amountwithin the range of a preset value for improving the image quality undera special condition to perform 100% UCR in gray in such a meaning. Suchprocessing has the advantage that the gray balance does not change ifsubtle balance change occurs in the gray level characteristic of C, Mand Y.

In JP-A-2005-33348, however, as seen in FIG. 7, correction coefficientf(k) is used in the range of f(k)>1 and thus if it is also used forlimiting the toner total amount in usual four-color separation to mainlyuse the range of f(k)<1, the f(k) range and resolution and memory andcircuit scale saving become difficult to be compatible.

In JP-A-2005-33348, for gray scale correction (γ correction) provided atthe later stage of four-color separation as seen in FIG. 1, if it isreleased to the user directly as a density (or brightness) adjustmentparameter, it becomes necessary to provide change caused by useradjustment as a margin in addition to the fluctuation of the engine forthe limitation value of the toner total amount. Thus, only the colormaterial less the margin than the essential capability of the printerengine can be used and the color reproduction range is narrowed.

SUMMARY OF THE INVENTION

The present invention has been made in view of above circumstances andprovides a color image processing apparatus and color printer system.According to an embodiment of the invention, the color printer systemfor printing with mixed CMYK color materials realizes total amountcontrol to limit the total color material amount of CMYK color mixing toa preset value or less or an image processing apparatus used with thecolor printer system, etc. According to an embodiment of the invention,there is provided a color printer system and an image processingapparatus (1) not requiring feedback of the output signal value for eachpixel, (2) saving the memory consumption amount and the computationamount, thereby decreasing the hardware installation cost, thecomputation processing time, and the data transfer time, and (3) alsomaking possible color material amount increasing processing in theregulation range of the total color material amount; particularlyrealizing processing wherein under color addition to prevent saturationdegradation in processing involving a large under color removal amountas in the case where 100% UCR is executed in gray and the toner totalamount control in usual four-color separation can be compatible witheach other in a small memory amount and computation amount and (4)furthermore preferably, not requiring feedback from user adjustmenteither.

According to an aspect of the present invention, there is provided acolor image processing apparatus that generates output gray level values(c4, m4, y4, k4) of four colors including black from input gray levelvalues (c1, m1, y1) of three colors of cyan, magenta and yellow. Thecolor image processing apparatus includes a minimum value selection unitthat selects a minimum value k0 of the input gray level values (c1, m1,y1); a first calculation unit that calculates a generation amount k3 ofblack based on the minimum value k0; a second calculation unit thatcalculates an under color removal amount u(k0) which is a value relevantto the minimum value k0; a third calculation unit that outputs acorrection coefficient value f(k0) which is a value relevant to theminimum value k0; a subtraction unit that subtracts the under colorremoval amount u(k0) from each of the input gray level values (c1, m1,y1) to generate output (c2, m2, y2); a multiplication unit that multiplythe output of the subtraction unit (c2, m2, y2) by the correctioncoefficient value f(k0) to obtain output (c3, m3, y3); a significant bitselection unit that selects a significant bit of a multiplication resultof the multiplication unit; and a gray level correction unit that makesa gray level correction to the generation amount k3 of black and theoutput of the multiplication unit (c3, m3, y3) to obtain the output graylevel values (c4, m4, y4, k4).

According to another aspect of the invention, there is provided thecolor image processing apparatus wherein an input/output relationship ofat least one of the first, second and third calculation units, and anoperation of the significant bit selection unit switches in conjunctionwith each other.

According to still another aspect of the invention, there is providedthe color image processing apparatus wherein the third calculation unitincludes an LUT and interpolation computation unit.

According to a further aspect of the invention, there is provided thecolor image processing apparatus wherein a range of an input value k0 ofthe third calculation unit is 8 bits, a range of an output value f(k0)of the third calculation unit is 10 bits or more, and a range of anoutput of the significant bit selection unit as an input to the graylevel correction unit is 8 bits.

According to a still further aspect of the invention, there is provideda color image processing apparatus for generating output gray levelvalues (c4, m4, y4, k4) of four colors including black from input graylevel values (c, m, y) of three colors of cyan, magenta and yellow, thecolor image processing apparatus comprising: a first gray levelcorrection unit that makes a gray level correction to the input graylevel values (c, m, y); a color correction unit that makes a colorcorrection to output values of the first gray level correction unit (c0,m0, y0) to obtain output (c1, m1, y1); a minimum value selection unitthat selects a minimum value k0 of the output (c1, m1, y1); a firstcalculation unit that calculates a generation amount k3 of black basedon the minimum value k0; a second calculation unit that calculates anunder color removal amount u(k0) which is a value relevant to theminimum value k0; a third calculation unit that outputs a correctioncoefficient value f(k0) which is a value relevant to the minimum valuek0; a subtraction unit that subtracts the under color removal amountu(k0) from the output (c1, m1, y1) to generate output (c2, m2, y2); amultiplication unit that multiplies the output of the subtraction unit(c2, m2, y2) by the correction coefficient value f(k0); a significantbit selection unit that selects a significant bit of a multiplicationresult of the multiplication unit; a second gray level correction unitthat makes a gray level correction to the generation amount k3 of blackand an output of the multiplication unit (c3, m3, y3) to obtain outputgray level values (c4, m4, y4, k4); and a user interface that allows auser to enter density gray level adjustment parameters of cyan, magenta,yellow, and black; wherein an input/output relationship of the firstgray level correction unit switches in accordance with the gray leveladjustment parameters of cyan, magenta, yellow; and wherein aninput/output relationship of the first calculation unit switches inaccordance with the gray level adjustment parameter of black.

According to embodiments of the invention, a system (a color printersystem including a host computer (PC) for editing and creating an imageand a printer of an image record section) can be implemented as a methodof installing the whole of the image processing section as software inthe PC, a method of installing the whole of the image processing sectionin a controller of the printer, or a method of installing somecomponents including the user interface and the first gray levelcorrection unit of the image processing section as software in the PC(driver section). In this case, the user interface is provided fordirectly prompting the user to make CMYK density adjustment, foradjusting the brightness of RGB (equivalent to adjusting of the densityof CMY of complement of RGB), or for adjusting the brightness signalcorresponding to the RGB value (arithmetically converting the brightnesssignal into adjustment of RGB signal). The density adjustment parameterconcerning K is added to image data from the driver section in the PCand is sent to the image processing section in the printer (controlsection) as required.

Other aspects of the invention will be more clearly understood by thefollowing description.

According to embodiments of the invention, the total amount control ofthe color materials is realized by multiplying the output of thesubtraction unit (c2, m2, y2) by the correction coefficient f(k0). Atthis time, the correction coefficient f(k0) is determined depending onlyon k0 of the minimum value of the CMY input value (c1, m1, y1), so thatthe separation result into CMYK (c4, m4, y4, k4) and the feedback fromthe printer engine is not required, the logic scale is also saved, andthe processing is easily speeded up.

Since the correction coefficient f(k0) can also be defined independentlyof user's density adjustment, so that in the printer system shared amongcomputers, a sequence for feedback each time a print request is madedoes not occur, and extra load on the apparatus and an increase in theoperation cost are prevented.

Since the correction coefficient f(k0) is defined as a functionconcerning the minimum value k0 of the CMY input value (c1, m1, y1) or aone-dimensional LUT, it is made possible to implement the correctioncoefficient f(k0) in a small memory consumption amount as compared withthe total amount control system using a multidimensional LUT.

Further, f(k0) is implemented as a one-dimensional LUT for therepresentative value of k0 and its implementation computation unit, sothat the memory consumption amount of the LUT can be still more savedand in addition, to rewrite the LUT, the data write time is shortenedand higher-speed processing can be covered.

In the invention, the selection unit that selects a significant bit outof the multiplication result of the multiplication unit can be switched,whereby it is made possible to switch the range in which the correctioncoefficient f(k0) acts on the subtraction unit output value (c2, m2, y2)between the range of f(k0)≦1 as main and the range of f(k0)>1 as main.Accordingly, both acting ranges can be covered in a narrow input rangeas the second gray level correction unit.

Accordingly, the toner total amount control in usual four-colorseparation and under color addition to prevent saturation degradation inprocessing involving a large under color removal amount as in the casewhere 100% UCR is executed in gray within the range of the total amountcontrol can be switched in a small memory amount and computation amount.

Particularly, output of the significant bit selection means is 8 bits,whereby it is made possible to share the general gray level correctionunit of the image processing unit as the second gray level correctionunit, and the apparatus development burden can be lightened.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in detail basedon the following figures, wherein:

FIG. 1 is a general schematic configuration drawing to show a colorlaser printer system according to an embodiment of the invention;FIG. 2is a schematic configuration drawing to show a four-color separationunit according to an embodiment of the invention;

FIG. 3 is a schematic representation of gray level correction accordingto an embodiment of the invention;

FIG. 4 is a characteristic drawing of input gray level value k0 vs. BGvalue and UCR value according to an embodiment of the invention;

FIG. 5 is a characteristic drawing of input gray level value k0 vs.correction coefficient f(k0) according to an embodiment of theinvention;

FIG. 6 is a characteristic drawing of input gray level value k0 vs.correction coefficient f(k0) according to an embodiment of theinvention;

FIG. 7 is a schematic configuration drawing to show a record sectionaccording to a first embodiment of the invention;

FIG. 8 is a schematic representation of a density adjustment userinterface according to an embodiment of the invention;

FIG. 9 is a schematic configuration drawing to show a record sectionaccording to a second embodiment of the invention;

FIG. 10 is a schematic representation of an image formation unit in FIG.9;

FIG. 11 is a schematic representation to show a configuration example ofcorrection coefficient generation unit;

FIG. 12 is a schematic representation to show another configurationexample of the correction coefficient generation unit;

FIG. 13 is a schematic configuration drawing to show an image processingsection in a printer system according to an embodiment of the invention;

FIG. 14 is a schematic representation of a density adjustment userinterface corresponding to the system in FIG. 13;

FIG. 15 is a flowchart to show a color image processing method accordingto an embodiment of the invention; and

FIG. 16 is a flowchart to show details of one step in FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the accompanying drawings, there are shown preferredembodiments of the invention.

FIG. 1 shows the general configuration of a color laser printer systemaccording to the invention. The color laser printer system is mainlymade up of a computer 10, a color image processing apparatus 11, and arecord section (printer engine) 13. An image processing section 12 ofthe system is made up of a driver processing section 18 and a userinterface 19 implemented as software in the computer 10 and the colorimage processing apparatus 11 installed as hardware in a controller ofthe printer. The color image processing apparatus 11 is made up of animage input section 11A and a color image processing section 11B. Theembodiments are characterized by the image processing section 12. Theconfigurations and functions of the sections will be discussed in order.

1. Computer 10

The computer 10 in the color laser printer system is, for example, apersonal computer (PC) to which information for giving a densityadjustment command to each color of CMYK is input through the userinterface 19.

A print instruction given to the color image processing apparatus 11 isexecuted through the printer driver 18 from an application of the PC 10.At this time, the image data to be printed is compressed and istransferred to the color image processing apparatus 11 together withadditional information 15 containing a user command input through theuser interface 19.

FIG. 8 shows an example of the density adjustment user interface 19provided in the PC 10. The user interface 19 is provided with densityadjustment sliders 81 in a one-to-one correspondence with C, M, Y, andK, enabling the user to specify a density adjustment parameter in any of21 steps in the range of −10 to +10 at the position of a marker 82 ofeach slider. The printer driver 18 adds the density adjustment parametervalues of C, M, Y, and K to the additional information 15 and transmitsthe additional information 15 to the image processing apparatus 11together with image data.

2. Input Section 11A

The input section 11A of the color image processing apparatus 11 has animage expansion unit 1 and an input buffer 2. The image expansion unit 1expands the image information sent from the driver 18 in the inputbuffer 2 and also sends the additional information 15 attached to theimage information to a parameter management unit 14. The image dataexpanded by the image expansion unit 1 is stored in the input buffer 2as one-page RGB data.

The image data stored in the input buffer 2 is read in succession and issent to the color image processing section 11B and is subjected toprocessing of color correction, gray level correction, etc., and then issent to the record section 13.

3. Record Section 13

Next, an outline of the record section 13 will be discussed.

FIG. 7 is a schematic configuration drawing to show an embodiment of therecord section 13. The record section 13 adopts an intermediate transferbody system of using a photoconductor 64 and an intermediate transferbody 66, transferring different four-color toner images formed one colorat a time in order on the photoconductor 64 one color at a time everyrevolution of the intermediate transfer body 66, and forming one colorimage with four revolutions of the intermediate transfer body 66. InFIG. 7, the photoconductor 64 shaped like a belt is placed in the centerof the record section 13 and the intermediate transfer body 66 is placedon one face of the photoconductor 64 in contact with the photoconductorbelt 64. Four developing machines for storing different color toners,namely, a yellow (Y) developing machine 63Y, a magenta (M) developingmachine 63M, a cyan (C) developing machine 63C, and a black (K)developing machine 63K are placed in a longitudinal stack manner on anopposite face of the photoconductor belt 64 stretching longitudinallylong to the intermediate transfer body 66.

The photoconductor belt 64 is surrounded by a charger 60, an exposureunit 62, the developing machines 63, etc., of process parts required forforming toner images on the photoconductor belt 64 along the rotationdirection of the photoconductor belt 64.

On the other hand, the intermediate transfer body 66 is surrounded by atransfer roller 67 of a process part required for forming toner imagesand transporting paper.

A transport passage for transporting paper is defined from a papercassette 69 placed in a lower part of a main unit through the outside ofthe intermediate transfer body 66 to the top face of the main unit, andthe transfer roller 67, a paper static eliminator (not shown), and afuser 59 are placed along the transport passage.

Next, the operation of the embodiment of the record section 13 will bedescribed as follows.

The photoconductor belt 64 and the intermediate transfer body 66 arerotated in a predetermined direction and the surface of thephotoconductor belt 64 is charged to a predetermined potential by thecharger 60. The charged surface of the photoconductor belt 64 is exposedto light by the exposure unit 62 that generates laser lightcorresponding to the image data. To expose the surface of thephotoconductor belt 64 to light, first an image of one of four colors,for example, a yellow (Y) image is exposed to light and an electrostaticlatent image corresponding to the yellow (Y) component is formed on thesurface of the photoconductor belt 64. Then, the developing machine 63Ystoring yellow (Y) toner corresponding to the electrostatic latent imageon the photoconductor belt 64 is brought into contact with thephotoconductor belt 64 to form a toner image, and the image formed onthe photoconductor belt 64 is transferred to the intermediate transferbody 66.

Next, a toner image of the second color, for example, a magenta (M)image is formed on the photoconductor belt 64 and the intermediatetransfer body 66 is rotated one revolution and then the M image istransferred onto the Y toner image of the first color reaching thecontact part with the photoconductor belt 64. After this, the process isrepeated for the third color, for example, cyan (C) and the fourthcolor, for example, black (K) to form an image with four color tonersoverlaid on the intermediate transfer body 66.

Last, the toner image formed on the intermediate transfer body 66 istransferred onto paper 68 fed from the paper cassette 69 by the transferroller 67. Electricity is removed by the paper static eliminator (notshown) and the paper is output through the fuser 59 to the outside ofthe main unit.

In the embodiment, a reference patch is formed on the photoconductorbelt 64 at a specific timing such as the engine starting time and thedensity of the formed patch is detected by an optical sensor 65. Adetection signal is sent to the color image processing section 11B(described later) as a sensor signal 16.

FIGS. 9 and 10 show the record section 13 according to anotherembodiment of the invention. The record section has four image formationunits 95Y, 95M, 95C, and 95K placed in a longitudinal stack manner inthe proximity of an intermediate recording medium 93. Each of the imageformation units has a photoconductor drum 85, a charger 86, an exposureunit 87, a developing machine 88, a first transfer unit 89, and acleaning unit 90, as shown in FIG. 10.

The photoconductor drum 85 provided by applying a coating of aphotoconductor to the outer peripheral surface of an aluminum cylinder,etc., is driven in the arrow direction and the surface of thephotoconductor drum 85 is uniformly charged to a predeterminedpotential.

When an image write signal is input to the exposure unit 87 from acontrol section 91, a laser is produced in response to the signal andlight information corresponding to the image signal is applied to thesurface of the photoconductor drum 85 for forming an electrostaticlatent image. Further, when the photoconductor drum 85 proceeds in thearrow direction, the electrostatic latent image is developed by thedeveloping machine 88 to form a toner visible image. The developed tonervisible image is transferred onto the intermediate recording medium 93by the first transfer unit 89 to which a predetermined bias voltage isapplied. Next, the intermediate recording medium 93 onto which the tonerimage is transferred is transported by a transport unit and the tonerimage is transferred to record paper 98 by a second transfer unit 96 andthen is fixed by the fuser 97 to form a permanent image.

Also in the embodiment, an optical sensor 92 is placed in the proximityof the intermediate recording medium 93 and a reference patch is formedon the intermediate recording medium 93 at the engine starting time,etc., and the density of the formed patch is detected by the opticalsensor 92. A detection signal is sent to the control section 91 as asensor signal 16.

4. Color Image Processing Section 11B

The color image processing section 11B in the color image processingapparatus 11 has a color correction unit 3, a four-color separation unit5, a gray level correction unit 7, a gray level processing unit 9, andthe parameter management unit 14. These units will be discussed below.

4.1 Color Correction Unit 3

The color correction unit 3 makes a color correction to RGB data (r, g,b) sent from the input buffer 2. For example, a multidimensional LUT(look-up table) for color correction is provided in the parametermanagement unit 14, and the color correction unit 3 loads the LUT(look-up table) and makes a color correction to the RGB data (r, g, b).The technique of the correction is already known and will not bediscussed in detail.

4.2 Four-Color Separation Unit 5

The four-color separation unit 5 is a unit that generates four-colordata of CMYK (c, m, y, k) from RGB data (r, g, b) and is mainly made upof a complement computation unit 21, a first gray level correction unit72, 73, 74, and 75, a UCR calculation unit 70, a BG calculation unit 71,a correction coefficient generation unit 31, etc., as shown in FIG. 2.

a. Complement Computation Unit 21

The complement computation unit 21 is a unit that generates CMY (c0, m0,y0) from RGB data (r, g, b) subjected to color correction. In theembodiment, each piece (component) of the RGB data (r, g, b) consists of8 bits. The data (c0, m0, y0) is found according to the followingexpression (2). $\begin{matrix}\left. \begin{matrix}{{c\quad 0} = {{I\quad\max} - r}} \\{{m\quad 0} = {{I\quad\max} - g}} \\{{y\quad 0} = {{I\quad\max} - b}}\end{matrix} \right\} & (2)\end{matrix}$where Imax denotes the maximum value represented by 8 bits, namely, 255.The computation in expression (2) can be executed by simple bitinversion. That is, the RGB data and the CMY data are equivalent datadifferent only in logic (positive or negative) and explicit thecomplement computation unit 21 can be omitted simply by reversing theinterpretation of 0, 1 signal. To simplify the description, the positivelogic is applied throughout the Specification.

Imax=255 is for illustrative purpose only and if each piece (component)of the RGB data (r, g, b) is made up of 16 bits for advanced saturationcomputation, Imax=2¹⁶=65535. Of course, the invention is not limited tothe value of Imax.

b. First Gray Level Correction Unit 72, 73, and 74

Each of the first gray level correction unit 72, 73, and 74 performsgray level correction processing called γ correction to the input imagedata while referencing LUT (γc, γm, γy).

The parameter management unit 14 sets LUT (γc, γm, γy) in accordancewith the additional information 15 previously added to the image databefore one-page processing.

The additional information 15 is parameter added to the image data bythe printer driver 18 through the user interface 19 and in theembodiment, is given as integer values (ic, im, iy, ik) in 21 steps of 0to 20 (default 11) for each color.

According to the integer values, the parameter management unit 14generates the LUT (γc, γm, γy) values based on the following relation,y=I max{1−(1−x/I max)^((1+(i−11)/10))}  (3)where i is the corresponding value of ic, im, iy, or ik, x=0-255 is theinput value referencing the table, and y is a table entry of the outputvalue. As the LUT generation method, another method of providing 21pieces of table data in external memory and selecting from among the 21pieces of table data in accordance with the additional information 15and setting the values in LUT (γc, γm, γy) is also available.

Here, data appearing in output of the first gray level correction unit72, 73, and 74 after the three-color CMY data (c0, m0, y0) is subjectedto γ correction is represented as (c1, m1, y1). The post-corrected data(c1, m1, y1) is added to a minimum value computation unit 22 for findingminimum value k0. k0 is represented by the following expression.k0=min{c1, m1, y}  (4)c. UCR Calculation Unit 70, BG Calculation Unit 71, and CorrectionCoefficient Generation Unit 31

The UCR calculation unit 70 references a UCR table (simply UCR) usingthe minimum value k0 as an index and outputs a UCR value. Here, thevalue provided by referencing the table UCR using the minimum value k0as an index is represented as u(k0).

Like the gray level correction unit 72, 73, 74, the first gray levelcorrection unit 75 makes a γ correction to k0 and the resultant k0 valueis added to the BG calculation unit 71. Letting output after the γcorrection be γk(k0), the BG calculation unit 71 references a BG table(which will be hereinafter referred to as simply BG) using γk(k0) as anindex and outputs a BG value k3.

To separate the function and simplify the description, the first graylevel correction unit 75 and the BG calculation unit 71 are describedseparately; however, as the tables are previously combined, it is easyfor the BG calculation unit 71 to also serve as the function of thefirst gray level correction unit 75.

The correction coefficient generation unit 31 generates a 10-bitcorrection coefficient value f(k0) for the 8-bit output value k0 of theminimum value computation unit 22.

To implement the correction coefficient generation unit 31, a method ofusing an LUT of 10-bit values directly corresponding to all values of 0to 255 of the input value k0 or a method of generating f(k0) accordingto a representative value and interpolation operation as shown in FIG.11 or 12 may be adopted. The latter method is effective for saving LUTmemory. If the LUT is a small table, the rewrite time for rewriting thetable contents is also shortened and thus the method is effective forspeeding up the processing.

FIG. 11 shows a configuration example of the correction coefficientgeneration unit 31. Letting the high-order 5 bits of the input value k0be kh and the low-order 3 bits be kL, kh is added to LUTs (look-uptables) 130 and 131. The LUTs 130 and 131 are referenced using kh as anindex. The value of f(kh) is preset in the entry of the LUT 130corresponding to kh and the value of f(kh+1) is preset in the entry ofthe LUT 131 corresponding to kh. In the figure, [7:3] means the 5 bitsof bits 3 to 7 of bits 0 to 8 and [2:0] means the 3 bits of bits 0 to 2.

A multiplication unit 132 inputs the output value f(kh) of the LUT 130and the value provided by inverting the kL value (2³-kL) and outputs theproduct of the values, a 13-bit value.

On the other hand, a multiplication unit 133 inputs the output valuef(kh+1) of the LUT 131 and kL and outputs the product of the values, a13-bit value. An addition unit 134 outputs the sum of the multiplicationunit 132 and 133 and the high-order 10 bits of the 13-bit output resultis adopted as the output value of the correction coefficient generationunit 31. The output value of the correction coefficient generation unit31 in FIG. 11 corresponds to the value y given according to thefollowing expression:y={(2³ −kL)·f(kh)+kL·f(kh+1)}/2³   (5)(fractional portion is dropped)

FIG. 12 shows another configuration example of the correctioncoefficient generation unit 31. The high-order 3 bits of the input valuek0, kh, is added to look-up tables 135 and 136. The value of f(kh) isset in the entry of the LUT 135 corresponding to kh and the value off(kh+1)=f(kh) is set in the entry of the LUT 136 corresponding to kh.

A multiplication unit 137 outputs a 13-bit value resulting frommultiplying the output value of the LUT 136 by kL and the high-order 10bits of the output result are input to addition unit 138, which thenadds the output value of the LUT 135 to the value of the high-order 10bits and the addition result is adopted as the output value of thecorrection coefficient generation unit 31.

The output value of the correction coefficient generation unit 31 inFIG. 12 corresponds to the value y given according to the followingexpression.y=f(kh)+{f(kh+1)−f(kh))·kL}/2³   (6)(fractional portion is dropped)

In this case, the value of the LUT 136 becomes negative; however, if fis sufficiently smooth, the difference value, g(kh)=f(kh+1)−f(kh) doesnot become a large absolute value and thus the number of the bits of theretained value can also be decreased for furthermore saving the memoryin the range in which there is no problem if the LUT 136 is handled as asigned integer.

Comparing the configuration of the correction coefficient generationunit 31 as in FIG. 11 or 12 with the case where an 8-bit input, 10-bitoutput LUT is directly implemented, in the direct implementation, ifdata is retained with 10-bit output matched with 8-bit boundaries, 16bits (2 bytes) become substantially necessary and thus 512 bytes becomenecessary; while, in the configuration in FIG. 11, 12, the two LUTs arepaired to form 20-bit data and data is handled in 24-bit=3-byte units,so that it is made possible to implement with 3×2⁵=96 bytes.

The composition method of the value of f(k0) implemented in the tablesis described later.

d. Subtraction Unit 25, 26, and 27 and Multiplication Unit 28, 29, and30

Output data of the first gray level correction unit 72, 73, 74 (c1, m1,y1) is added as one input of subtraction unit 25, 26, 27, and outputu(k0) of the UCR calculation unit 70 is added as the other input.Therefore, output data of the subtraction unit 25, 26, 27 (c2, m2, y2)is represented by the following expression (7). $\begin{matrix}\left. \begin{matrix}{{c\quad 2} = {{c\quad 1} - {u\quad\left( {k\quad 0} \right)}}} \\{{m\quad 2} = {{m\quad 1} - {u\quad\left( {k\quad 0} \right)}}} \\{{y\quad 2} = {{y\quad 1} - {u\quad\left( {k\quad 0} \right)}}}\end{matrix} \right\} & (7)\end{matrix}$

Output of the subtraction unit 25, 26, 27 (c2, m2, y2) is added tomultiplication unit 28, 29, 30, and 10-bit output f(k0) of thecorrection coefficient generation unit 31 is added as the other input.Therefore, output data of the multiplication unit 28, 29, 30 (c3, m3,y3) is represented by the following expression (8): $\begin{matrix}\begin{matrix}{{c\quad 3} = {f\quad{\left( {k\quad 0} \right) \cdot c}\quad 2}} \\{{m\quad 3} = {f\quad{\left( {k\quad 0} \right) \cdot m}\quad 2}} \\{{y\quad 3} = {f\quad{\left( {k\quad 0} \right) \cdot y}\quad 2}}\end{matrix} & (8)\end{matrix}$e. Effective Bit Selection Unit

In the embodiment, the output of the subtraction unit 25, 26, 27 is 8bits and the output of the correction coefficient generation unit 31 is10 bits. At this time, the computation result of expression (8) becomes18 bits. To implement the second gray level correction unit 7 as aone-dimensional LUT, whenever the input range increases one bit, thenecessary number of table entries becomes double and therefore itbecomes necessary to set the input range to the minimum necessary numberof bits to save memory.

Generally, if there is no loss of resolution caused by gray levelcorrection, it is considered that 8 bits are necessary and sufficientfor continuous tone representation and therefore it is considered thatthe optimum input range of the second gray level correction unit 7 forthe purpose of linearizing the print density (more precisely,linearizing with respect to the color material amount (toner amount) toprint) as described later is 8 bits.

Thus, according to a selection signal 35, effective bit selection unit32, 33, 34 selects intermediate 8 bits of the 18-bit multiplicationresult of the multiplication unit 28, 29, 30 with the low-order 8 bitsand the high-order 2 bits discarded (in FIG. 2, [15:8] means theselected signal range) if the selection signal 35 is 0, and selectsintermediate 8 bits with the low-order 7 bits and the high-order 3 bitsdiscarded ([14:7]) if the selection signal 35 is 1.

Such switching is performed, whereby if 0 is specified in the selectionsignal 35, the correction coefficient f(k0) in expression (8) acts onthe output value of the subtraction unit 25, 26, 27 (c2, m2, y2) as afixed point number of resolution with 2 bits to the left of the decimalpoint and 8 bits to the right of the decimal point (0 to 3+254/255); if1 is specified in the selection signal 35, the correction coefficientf(k0) acts on (c2, m2, y2) as a fixed point number of resolution with 3bits to the left of the decimal point and 7 bits to the right of thedecimal point (0 to 7+127/128).

However, the high-order bits are also discarded in the effective bitselection unit 32, 33, 34 and thus the output is clipped to the 8-bitrange of 0 to 255; in this point, the value range of f(k0) is limited sothat the multiplication result does not exceed 255 as described later asthe f(k0) composition method and therefore no problem arises.

Accordingly, as implementation of f(k0), processing of assigningpriority to the resolution and accurately making a narrow-rangecorrection and processing of slightly sacrificing the resolution andmaking a correction adaptable to a wider range are made compatible witheach other with the same (10-bit) bit width by switching the selectionsignal 35; it is made possible to enlarge the range of f(k0) responsiveto the purpose while saving the logic scale.

Particularly, to make the correction coefficient f(k0) act mainly forthe purpose of reducing the value of (c2, m2, y2) (f(k0)<1 is main), thevalue 0 is specified in the selection signal 35; to make the correctioncoefficient f(k0) act for the purpose of enlarging the value of (c2, m2,y2) (f(k0)>1 is main), the value 1 is specified in the selection signal35, whereby the optimum correction can be made.

f. Selection Unit 23

The output data of the multiplication unit 28, 29, 30 (c3, m3, y3) andoutput data k3 of the BG calculation unit 71 are added to selection unit23. The selection unit 23 selects one of four pieces of input data inresponse to a selection signal 24 sent from a control section (notshown) and sends the selected input data to the second gray levelcorrection unit 7 at the following stage. The selection signal 24 is asignal switched in synchronization with a vertical synchronizing signalof the printer for selecting a color signal responsive to the outputcolor at the time as output from among four color data pieces (k3, c3,m3, y3).

4.3 Second Gray Level Correction Unit 7 and Gray Level Processing Unit 9

The second gray level correction unit 7 calibrates the nonlinear graylevel characteristic determined by the characteristic of the recordsection 13 and the gray level processing method of the gray levelprocessing unit 9 as shown in a in FIG. 3, for example, as an almostlinear characteristic as in b in FIG. 3 and particularly makes acorrection so that the relationship between the input image data (k3,c3, m3, y3) and the toner deposition amount of the output image becomesalmost linear.

Although the output value of the second gray level correction unit 7 isusually 8 bits, it is also effective to relate an output value exceeding8 bits to a strongly nonlinear characteristic like the characteristic ina in FIG. 3 to compensate for the gradation impaired by correction.

To handle the colors of CMYK switched by the selection unit 23, look-uptables γc′, γm′, γy′, γk′ for correction are provided in the parametermanagement unit 14, and the second gray level correction unit 7previously loads the look-up table according to the selection signal 24and then makes a correction to the input data while referencing thetables.

The gray level processing unit 9 performs processing of relating theoutput value of the second gray level correction unit 7 to an ON/OFFpulse that can be output in the record section 13, and sends the outputsignal to the control section 91 of the record section 13.

Next, the correction coefficient f(k0) generation method of thecorrection coefficient generation unit 31 will be discussed in detail.

4.4 Correction Coefficient f(k0) Generation Method

If “0” is selected as the selection signal 35 in FIG. 2, the 10-bitcorrection coefficient value f(k0) is generated as a value selected fromamong 0 to 3+255/256% 3.996 in 1/256 steps and acts on the output valueof the subtraction unit 25, 26, 27 (c2, m2, y2) as previously described.If “1” is selected as the selection signal 35 in FIG. 2, f(k0) isgenerated as a value selected from among 0 to 7+127/128% 7.99 in 1/128steps and acts on the output value of the subtraction unit 25, 26, 27(c2, m2, y2) as previously described.

In the description to follow, it is assumed that any range mentionedabove is selected as the f(k0) range and the multiplication result basedon f(k0) is stored in the output range of 0 to Imax=255 by extraction ofintermediate 8 bits described above. In the description to follow, it isassumed that the input/output range is unified to 0 to Imax.

At this time, the conditions required for the correction coefficientf(k0) become the following three conditions for (c3, m3, y3, k3):

Condition 1: f(k0) does not exceed a magnification upper limit valueAmax.

Condition 2: The output range of (c3, m3, y3) does not exceed themaximum value Imax.

Condition 3: The total of (c3, m3, y3, k3) does not exceed a total tonerlimit value Tmax.

The magnification upper limit Amax in condition 1 is a constant suchthat Amax<1023/128≈77.99 from the assumption concerning theinterpretation of f(k0) mentioned above.

As for condition 2, for example, for c3c3=f(k0)·c2≦f(k0)(I max−u(k0))≦I max (9) Therefore,$\begin{matrix}{{f\quad\left( {k\quad 0} \right)} \leq \frac{I\quad\max}{{I\quad\max} - {u\quad\left( {k\quad 0} \right)}}} & (10)\end{matrix}$condition 2 is represented as above.

Likewise, as for condition 3, assuming that the output data of (c3, m3,y3, k3) of the four-color separation unit 5 is calibrated so as tobecome proportional to the toner amount according to the gray levelcorrection table y in the second gray level correction unit 7, the casewhere two colors of c1, m1, and y1 are Imax is considered, whereby thetotal toner limit value Tmax is represented by the following expression(11): $\begin{matrix}\begin{matrix}{S = \left( {{c\quad 3} + {m\quad 3} + {y\quad 3} + {k\quad 3}} \right)} \\{= {{{f\quad\left( {k\quad 0} \right)\quad\left( {{c\quad 2} + {m\quad 2} + {y\quad 2}} \right)} + {k\quad 3}} \leq {{f\quad\left( {k\quad 0} \right)\quad\left( {{I\quad\max} - {u\quad\left( {k\quad 0} \right)}} \right)} +}}} \\{{f\quad\left( {k\quad 0} \right)\quad\left( {{I\quad\max} - {u\quad\left( {k\quad 0} \right)}} \right)} + {f\quad\left( {k\quad 0} \right)\quad\left( {{k\quad 0} - {u\quad\left( {k\quad 0} \right)}} \right)} + {k\quad 3}} \\{= {{{f\quad\left( {k\quad 0} \right)\quad\left\{ {{2\quad\left( {{I\quad\max} - {u\quad\left( {k\quad 0} \right)}} \right)} + \left( {{k\quad 0} - {u\quad\left( {k\quad 0} \right)}} \right)} \right\}} + {k\quad 3}} \leq {T\quad\max}}}\end{matrix} & (11)\end{matrix}$Therefore, $\begin{matrix}{{f\quad\left( {k\quad 0} \right)} \leq \frac{{T\quad\max} - {k\quad 3}}{2\quad\left( {{I\quad\max} - {u\quad\left( {k\quad 0} \right)} + {k\quad 0} - {u\quad\left( {k\quad 0} \right)}} \right)}} & (12)\end{matrix}$becomes the same value as condition 3.Thus, when $\begin{matrix}{{T\quad\left( {k\quad 0} \right)} = \frac{{T\quad\max} - {k\quad 3}}{2\quad\left( {{I\quad\max} - {u\quad\left( {k\quad 0} \right)} + {k\quad 0} - {u\quad\left( {k\quad 0} \right)}} \right)}} & (13)\end{matrix}$is set, condition 3 becomes expression (16). Condition 1 is representedas expression (14) and condition 2 is represented as expression (15).$\begin{matrix}{{f\quad\left( {k\quad 0} \right)} \leq {A\quad\max}} & (14) \\{{f\quad\left( {k\quad 0} \right)} \leq \frac{I\quad\max}{{I\quad\max} - {u\quad\left( {k\quad 0} \right)}}} & (15) \\{{F\quad\left( {k\quad 0} \right)} \leq {T\quad\left( {k\quad 0} \right)}} & (16)\end{matrix}$

Hereinafter, expression (14) will be referred to as the maximummagnification condition, expression (15) as the output range condition,and expression (16) as the maximum toner amount condition.

The maximum magnification condition (14) is a condition required whenextreme under color removal with the under color removal ratiou(k0)/Imax reaching about 90% or more is executed Usually, the outputrange condition (15) becomes the sufficient condition of the maximummagnification condition (14).

In fact, the relation between the right sides of the conditions (14) and(15) becomes $\begin{matrix}{{A\quad\max} \leq \frac{I\quad\max}{{I\quad\max} - {u\quad\left( {k\quad 0} \right)}}} & (17)\end{matrix}$when the under color removal ratio is $\begin{matrix}{\frac{u\quad\left( {k\quad 0} \right)}{I\quad\max} \geq {1 - \frac{1}{A\quad\max}}} & (18)\end{matrix}$For example, if Amax=15, the right side of expression (18) is about 0.93and therefore if the under color removal ratio does not exceed 93%, themaximum magnification condition (14) is automatically satisfiedaccording to the output range condition (15).

Since f(k0) needs to satisfy all of the maximum magnification condition(14), the output range condition (15), and the maximum toner amountcondition (16), f(k0) is found according to the following expression(19): $\begin{matrix}{{{f\quad\left( {k\quad 0} \right)} = \left\{ {{A\quad\max},\frac{I\quad\max}{{I\quad\max} - {u\quad\left( {k\quad 0} \right)}},{T\quad\left( {k\quad 0} \right)}} \right\}}\left( {{{k\quad 0} = 0},1,2,\ldots\quad,{I\quad\max}} \right)} & (19)\end{matrix}$

Next, specific examples of the BG table value BG(k0), the UCR tablevalue u(k0), and the correction coefficient table value f(k0) are shown.

FIG. 4 is a graph to show examples of the BG table value BG(k0) (=k3)and the UCR table value u(k0). The horizontal axis indicates the inputgray level value k0 and the vertical axis indicates the output graylevel value y. Curves 40 and 41 correspond to y=BG(k0) and y=u(k0)respectively. FIG. 5 shows an example of the correction coefficienttable value f(k0) obtained according to expression (19) relative to theBG table value BG(k0) and the UCR table value u(k0).

A curve group 42 in FIG. 5 is a graph of y=f(k0) when the maximum toneramount limit Tmax in 0.5 steps of 1.0 to 4.0 as the ratio of Tmax toImax (Tmax/Imax). Particularly, a curve 44 represents a graph of y=f(k0)when the maximum toner amount limit is set to 250% of Imax, namely,Tmax/Imax=2.5. A curve 43 is a graph of y=Imax/(Imax−u(k0)).

Therefore, a common area 46 below the curves 43 and 44 becomes an areasatisfying the conditions (14) to (16) under the limitation of themaximum toner amount 250% in BG and UCR setting in FIG. 4. A thick line45 of the upper limit of the common area 46 represents the curve y=f(k0)obtained according to expression (19) at this time.

In this example, if the maximum toner amount limit Tmax slightlychanges, f(k0) acts mostly as a value of f(k0)<1. Thus, if “0” isspecified as the selection signal 35 in FIG. 2 and f(k0) is set to avalue selected in the range of 0 to 3+255/256≈3.996 in 1/256 steps andis made to act on the output value of the subtraction unit 25, 26, 27(c2, m2, y2), the setting becomes the optimum setting for making therange of f(k0) and the resolution compatible with each other.

Of course, the conditions (14) to (16) would be satisfied in a hatchedarea 46 below the curve 45 and thus g(k0) such that g(k0)≦f(k0) maybeadopted as the value of the correction coefficient table of thecorrection coefficient generation unit 31. However, f(k0) is the bestsolution for making possible the widest color reproduction in the senseof giving the maximum value of the area 46.

Next, FIG. 6 shows an example wherein the BG table value BG(k0) and theUCR table value u(k0) are set to BG(k0)=u(k0)=k0. A curve group 47 inFIG. 6 is a graph of y=f(k0) when Tmax/Imax is in 0.5 steps of 1.0 to4.0, and a curve 48 particularly represents a graph of y=f(k0) whenTmax/Imax=2.5. A line 49 is y=Amax when the magnification upper limitAmax=8, and a curve 50 represents the output range limitation$\begin{matrix}{y = \frac{I\quad\max}{{I\quad\max} - {k\quad 0}}} & (20)\end{matrix}$

Therefore, a common area 52 hatched below the curves and line 48, 49,and 50 represents an area satisfying the conditions (14) to (16) in 100%UCR under the condition of the magnification upper limit 8X and themaximum toner amount 250%. Particularly, a thick line 51 of the upperlimit of the common area 52 corresponds to y=f(k0) obtained according toexpression (19).

In this example, f(k0) acts mostly as a value of 1 to 8 and thus if “1”is specified as the selection signal 35 in FIG. 2 and f(k0) is set to avalue selected in the range of 0 to 7+127/128% 7.99 in 1/128 steps andis made to act on the output value of the subtraction unit 25, 26, 27(c2, m2, y2), it is suited for making the range of f(k0) and theresolution compatible with each other (7+127/128 is used as f(k0)=8).

As described above, the correction coefficient table value f(k0) isgenerated according to expression (19) in response to the BG table valueBG(k0) and the UCR table value u(k0), four-color separation notexceeding the maximum toner amount Tmax is made possible. However, theBG table value BG(k0) depends on the gray level correction table γk ofthe first gray level correction unit 75 and therefore whenever thedensity adjustment command value for black is changed in the additionalinformation 15, it becomes necessary for the parameter management unit14 to again generate the correction coefficient table value f(k0) andagain load the value into the correction coefficient table of thecorrection coefficient generation unit 31.

This leads to the following problem: In a printer system shared betweenor among computers, the adjustment of the first gray level correctionunit made according to the favorite of each user becomes the setting inthe side (printer side) close to the record section 13 shared among theusers and thus if print jobs of the users are involved, the loads on theBG calculation unit 71, the UCR calculation unit 70, the correctioncoefficient generation unit 31, etc., for table calculation andreloading involved in reconstruction of LUT increase.

To circumvent this problem, another embodiment of the invention shown inFIG. 13 is an example wherein the color correction unit 3 and the firstgray level correction unit 72, 73, and 74 are removed from the colorimage processing section 11B of the printer and are implemented in theprinter driver 18 as software processing.

In the embodiment, the first gray level correction unit 72, 73, and 74are placed preceding the color correction unit 3. RGB input valuer, g, bis subjected to gray level correction in response to the value setthrough the user interface 19. In this case, as combined with a userinterface as gray level correction to the input RGB value, the userinterface free of a sense of being out of place for abolishing densityadjustment to K is constructed. If the density adjustment to K in theuser interface 19 is abolished, the first gray level correction unit 75for K can also be abolished.

In this case, the BG table value BG(k0), the UCR table value u(k0), andthe correction coefficient table value f(k0) can be previouslydetermined independently of the user setting for the user interface 19,so that it is made possible to circumvent the problem described above.

FIG. 14 shows an example of the user interface 19 corresponding to theexample in FIG. 13. User's density adjustment is covered by a densityadjustment slider 140 as a representative. If the user selects one ofinteger values in 21 steps of −10 to 10 as a setup value i with a marker144 of the density adjustment slider 140, according to the relationy=I max{(x/I max)^((1+(i/20)))}  (21)the first gray level correction unit 72, 73, and 74 generate LUTs andconvert all values of RGB input value (r, g, b) all together, wherebylight and shade of the whole image are converted. If a densityadjustment slider 141 for R is set, conversion of expression (21) forthe setup value with a marker 145 as well as the expression of theinput/output relation based on the density adjustment slider 140 settingis executed for the LUT of the first gray level correction unit 72,whereby the color balance concerning R is corrected. In this case, ifthe user sets the marker 145 to the “+” side, redness is increased andif the user sets the marker 145 to the “−” side, redness is decreased,whereby the density feeling of cyan of a complementary color isstrengthened. The same thing can be said for the operation of densityadjustment sliders 142 and 143 and markers 146 and 147 for other colors.

In the example of the user interface 19 in FIG. 14, a radio button 148for the user to specify switching of the black (K) replacement amount isprovided, enabling the user to select STANDARD or MAXIMUM exclusively.

In the embodiment in FIG. 13, as sets of the BG table value, the UCRtable value, and the correction coefficient table value, a “standardsetting” set made up of the BG and UCR table values shown in FIG. 4 andthe correction coefficient table value indicated by the curve 45 in FIG.5 and a “100% UCR setting” set made up of the correction coefficienttable values indicated by the curve 51 in FIG. 6 as BG(k0)=u(k0)=k0 arepreviously retained in the parameter management unit 14.

If the user selects STANDARD with the radio button 148 of the userinterface 19, the table values in the “standard setting” set are set inthe LUTs of the BG calculation unit 71, the UCR calculation unit 70, andcorrection coefficient generation unit 31 in FIG. 13, and 0 is set inthe value of a selection signal 35.

Likewise, if the user selects MAXIMUM with the radio button 148 of theuser interface 19, the table values in the “100% UCR setting” set areset in the LUTs of the BG calculation unit 71, the UCR calculation unit70, and the correction coefficient generation unit 31 in FIG. 13, and 1is set in the value of the selection signal 35. These settings arechanged all at once by the parameter management unit 14 based on theadditional information 15 added to image data through the user interface19.

If the user interface in FIG. 14 is not provided with the user interfacefor the user to enter a switching command of the black (K) replacementamount (in this case, the radio button 148), the adjustment of the firstgray level correction unit made according to the favorite of each useris completely closed in the adjustment of the computer of the user, sothat occurrence of the load of again generating the table caused by theuser adjustment is completely prevented.

However, if the user interface for the user to enter a switching commandof the black (K) replacement amount is simply provided as describedabove, the table generation calculation each time user adjustment ismade can be made unnecessary by providing all required table sets.

Particularly, in the “100% UCR setting,” switching unit for allowingprocessing to pass through is implemented for the UCR calculation unit70 and the BG calculation unit 71, whereby table loading can be passedand thus the table loading burden caused by switching becomessubstantially only the table setting for the correction coefficientgeneration unit 31.

Further, the correction coefficient generation unit 31 is implemented bythe interpolation method capable of saving the table data amount asdescribed above, so that the decrease effect of the table switching loadclose to the case where the black (K) replacement amount is not switchedcan be provided.

5. Image Processing Method

Next, one embodiment of an image processing method of the invention willbe discussed with a flowchart of FIG. 15. First, at step 100, theadditional information 15 added to image data is read. At step 101, thegray level correction tables γc, γm, γy, and γk in the first gray levelcorrection unit 72, 73, 74, and 75 are initialized based on theadditional information 15. Likewise, at step 102, sensor information 16concerning the toner deposition amount from the printer engine 13 isread and at step 103, the gray level correction tables γc′, γm′, γy′,and γk′ in the second gray level correction unit 7 are initialized basedon the sensor information 16.

To initialize the gray level correction tables of the first gray levelcorrection unit 72, 73, 74, and 75 and the second gray level correctionunit 7, tables are selected from among the gray level correction tablesprovided in the parameter management unit 14 according to the additionalinformation 15 or the sensor information 16 and are loaded.

Next, at step 104, the table UCR in the UCR calculation unit 70, thetable BG in the BG calculation unit 71, and the table f in thecorrection coefficient generation unit 31 are initialized.

Further, at step 105, the first pixel data (r, g, b) is input. At step106, complement data (c0, m0, y0) of the data (r, g, b) is calculatedaccording to expression (3) and then the gray level correction tablesγc, γm, and γy in the first gray level correction unit 72, 73, and 74are referenced using (c0, m0, y0) as an index, whereby data subjected togray level correction (c1, m1, y1) is obtained. c1, m1, and y1 arerepresented by expression (22): $\begin{matrix}\left. \begin{matrix}{{c\quad 1} = {\gamma\quad c\quad\left( {{I\quad\max} - r} \right)}} \\{{m\quad 1} = {\gamma\quad m\quad\left( {{I\quad\max} - g} \right)}} \\{{y\quad 1} = {\gamma\quad y\quad\left( {{I\quad\max} - b} \right)}}\end{matrix} \right\} & (22)\end{matrix}$

The minimum value computation unit 22 calculates minimum value k0 of c1,m1, y1. That is k0=min{c1, m1, y1}.

At step 107, the table UCR of the UCR calculation unit 70 is referencedusing k0 as an index and u(k0) is obtained. The table BG of the BGcalculation unit 71 is referenced and BG(k0) is obtained. The table f ofthe correction coefficient generation unit 31 is referenced and f(k0) isobtained.

At step 108, c3, m3, and y3 are calculated according to expressions (7)and (8). At step 109, the gray level correction tables γc′, γm′, γy′,and γk′ in the second gray level correction unit 7 are referenced using(c3, m3, k3) as an index, whereby (c4, m4, y4, k4) is acquired and atstep 110, is output. At step 111, whether or not processing for allpixels of one page is complete is determined. If the processing is notcomplete, the process returns to step 105 and the processing isrepeated.

FIG. 16 shows details of step 104 in FIG. 15. First, at step 120, thetable UCR of the UCR calculation unit 70 and the table BG of the BGcalculation unit 71 are initialized. To initialize the tables UCR andBG, they are loaded from a look-up table provided in the parametermanagement unit 14 as fixed values.

At step 121, index k0 is initialized to k0=0. At step 122, the tablesUCR and BG are referenced using k0 as an index and u(k0) and BG(k0) areobtained from the UCR calculation unit 70 and the BG calculation unit71. However, for the convenience of the description, γk(k0)=k0 andBG(k0)=k3.

At step 123, a comparison is made between u(k0) and Imax and ifu(k0)≠Imax, the process goes to step 124. At step 124, the correctioncoefficient value f(k0) is calculated according to expression (17).

On the other hand, if u(k0)=Imax at step 123, the process goes to step125 and f(k0) is set to 0.

At step 126, whether or not k0>Imax is determined. If NO is returned,the process goes to step 127, k0 is incremented by one, the processreturns to step 122, and the above-described processing is repeated.When k0 becomes greater than Imax, the processing is terminated.

Although one embodiment of the invention has been described, variousmodifications can be easily made within the scope of the basic conceptof the invention and these modifications are also contained in theinvention, of course.

To use the configuration shown in FIG. 9 as the record section 13, theselection unit 23 in FIG. 9 becomes unnecessary and the output data ofthe multiplication unit 28, 29, and 30 and the BG calculation unit 71(c3, m3, y3, k3) is added to the second gray level correction unit 7 andthe output data of the second gray level correction unit 7 (c4, m4, y4,k4) is added to the C unit 95C, the M unit 95M, the Y unit 95Y, and theK unit 95K in FIG. 9.

The first gray level correction unit 72, 73, and 74 and the colorcorrection unit 3 in FIG. 13 can also be implemented with the processingorder intact at the preceding stage of the complement computation unit21 in the four-color separation unit 5 as with the case in FIG. 2 ratherthan as the processing of the printer driver 18. In this case, the userinterface shown in FIG. 14 can be used as the user interface 19 and inaddition, the color correction unit 3 can be processed at high speed ashardware rather than software, but it becomes impossible to close theprocessing corresponding to the user adjustment in the computer.

The entire disclosure of Japanese Patent Application No. 2005-055326filed on Mar. 1, 2005 and Japanese Patent Application No. 2005-339550filed on Nov. 25, 2005 including specification, claims, drawings andabstract is incorporated herein be reference in its entirety.

1. A color image processing apparatus that generates output gray levelvalues (c4, m4, y4, k4) of four colors including black from input graylevel values (c1, m1, y1) of three colors of cyan, magenta and yellow,the color image processing apparatus comprising: a minimum valueselection unit that selects a minimum value k0 of the input gray levelvalues (c1, m1, y1); a first calculation unit that calculates ageneration amount k3 of black based on the minimum value k0; a secondcalculation unit that calculates an under color removal amount u(k0)which is a value relevant to the minimum value k0; a third calculationunit that outputs a correction coefficient value f(k0) which is a valuerelevant to the minimum value k0; a subtraction unit that subtracts theunder color removal amount u(k0) from each of the input gray levelvalues (c1, ml, y1) to generate output (c2, m2, y2); a multiplicationunit that multiply the output of the subtraction unit (c2, m2, y2) bythe correction coefficient value f(k0) to obtain output (c3, m3, y3); asignificant bit selection unit that selects a significant bit of amultiplication result of the multiplication unit; and a gray levelcorrection unit that makes a gray level correction to the generationamount k3 of black and the output of the multiplication unit (c3, m3,y3) to obtain the output gray level values (c4, m4, y4, k4).
 2. Thecolor image processing apparatus according to claim 1, wherein aninput/output relationship of at least one of the first, second and thirdcalculation units, and an operation of the significant bit selectionunit switches in conjunction with each other.
 3. The color imageprocessing apparatus according to claim 1, wherein the third calculationunit includes a look-up table and an interpolation computation unit. 4.The color image processing apparatus according to claim 3, wherein arange of an input value k0 of the third calculation unit is 8 bits, arange of an output value f(k0) of the third calculation unit is 10 bitsor more, and a range of an output of the significant bit selection unitas an input to the gray level correction unit is 8 bits.
 5. A colorimage processing apparatus for generating output gray level values (c4,m4, y4, k4) of four colors including black from input gray level values(c, m, y) of three colors of cyan, magenta and yellow, the color imageprocessing apparatus comprising: a first gray level correction unit thatmakes a gray level correction to the input gray level values (c, m, y);a color correction unit that makes a color correction to output valuesof the first gray level correction unit (c0, m0, y0) to obtain output(c1, m1, y1); a minimum value selection unit that selects a minimumvalue k0 of the output (c1, m1, y1); a first calculation unit thatcalculates a generation amount k3 of black based on the minimum valuek0; a second calculation unit that calculates an under color removalamount u(k0) which is a value relevant to the minimum value k0; a thirdcalculation unit that outputs a correction coefficient value f(k0) whichis a value relevant to the minimum value k0; a subtraction unit thatsubtracts the under color removal amount u(k0) from the output (c1, m1,y1) to generate output (c2, m2, y2); a multiplication unit thatmultiplies the output of the subtraction unit (c2, m2, y2) by thecorrection coefficient value f(k0); a significant bit selection unitthat selects a significant bit of a multiplication result of themultiplication unit; a second gray level correction unit that makes agray level correction to the generation amount k3 of black and an outputof the multiplication unit (c3, m3, y3) to obtain output gray levelvalues (c4, m4, y4, k4); and a user interface that allows a user toenter density gray level adjustment parameters of cyan, magenta, yellow,and black; wherein an input/output relationship of the first gray levelcorrection unit switches in accordance with the gray level adjustmentparameters of cyan, magenta, yellow; and wherein an input/outputrelationship of the first calculation unit switches in accordance withthe gray level adjustment parameter of black.
 6. A color printer systemcomprising: a computer that creates and edits RGB image data describedin three primary colors of red, green and blue; an image processingsection that processes the RGB image data; and a record section thatrecords a color image on a record medium based on the data processed bythe image processing unit, wherein the image processing unit comprises:a minimum value selection unit that selects a minimum value k0 ofcomplement data (c1, m1, y1) of the RGB image data; a first calculationunit that calculates a generation amount k3 of black based on theminimum value k0; a second calculation unit that calculates an undercolor removal amount u(k0) which is a value relevant to the minimumvalue k0; a third calculation unit that outputs a correction coefficientvalue f(k0) which is a value relevant to the minimum value k0; asubtraction unit that subtracts the under color removal amount u(k0)from the complement data (c1, m1, y1) to generate output (c2, m2, y2); amultiplication unit that multiplies the output of the subtraction unit(c2, m2, y2) by the correction coefficient value f(k0); a significantbit selection unit that selects a significant bit of a multiplicationresult of the multiplication unit; and a second gray level correctionunit that makes a gray level correction to the generation amount k3 ofblack and an output of the multiplication unit (c3, m3, y3) to obtainoutput gray level values (c4, m4, y4, k4).
 7. The color printer systemaccording to claim 6, wherein an input/output relationship of at leastone of the first, the second, and the third calculation units, and anoperation of the significant bit selection unit switches in conjunctionwith each other.
 8. The color printer system according to claim 6,wherein the correction coefficient value f(k0) includes a look-up tableand an interpolation computation unit.
 9. The color printer systemaccording to claim 8, wherein a range of an input value k0 of the thirdcalculation unit is 8 bits, a range of an output value f(k0) of thethird calculation unit is 10 bits or more, and a range of an output ofthe significant bit selection unit as an input to the second gray levelcorrection unit is 8 bits.
 10. The color printer system according toclaim 6 wherein the image processing section further comprises, a firstgray level correction unit that makes a gray level correction to the RGBimage data; and a density adjustment user interface that allows a userto enter a density adjustment parameter of density or brightness of aninput image; wherein the input/output relationship of the first graylevel correction unit switches in accordance with the density adjustmentparameter.
 11. The color printer system according to claim 10, whereinthe image processing section comprises: a driver section implemented assoftware in the computer, the driver section including the densityadjustment user interface and the first gray level correction unit; anda controller section which is hardware installed in the proximity of therecord section, the controller section including the second gray levelcorrection unit.
 12. The color printer system according to claim 11,wherein a density adjustment parameter concerning black is added toimage data transmitted from the driver section of the image processingsection to the control section.